柴北缘苦水泉金矿英云闪长岩和细粒闪长岩年代学、地球化学和Hf同位素及地质意义

Geochronology,geochemistry,Hf isotope,and their geological significance of the tonalite and fine-grained diorite from Kushuiquan gold deposit,North Qaidam

苦水泉金矿床位于柴北缘构造带中段,是近年来新发现的金矿床。该矿床具有造山型金矿的特征,矿体沿断裂构造分布在英云闪长岩中,空间上与细粒闪长岩脉密切相关。本文对苦水泉金矿中的英云闪长岩和细粒闪长岩进行了地球化学、锆石U-Pb定年和Hf同位素研究。全岩地球化学分析显示,英云闪长岩具有富钠贫钾(Na2O/K2O=6.24~13.09)、高Sr低Y(Sr/Y=205~335)的埃达克岩的特征,与锡铁山榴辉岩中的埃达克质浅色脉体十分相似;细粒闪长岩富铝、钙、铁,贫镁,富集轻稀土(LREEs)和大离子亲石元素(LILEs),贫高场强元素(HFSEs),Ni、Co含量低,为典型的大陆下地壳来源的岩石。锆石U-Pb定年显示,英云闪长岩和细粒闪长岩分别形成于429.9±2.5Ma和428.0±1.9Ma,Hf同位素分析显示英云闪长岩锆石εHf(t)值为+9.8~+11.9,二阶段模式年龄(tDM2)为613~747Ma,细粒闪长岩锆石εHf(t)值为-31.4~-9.9,二阶段模式年龄(tDM2)为1722~2803Ma。综合分析表明柴北缘在早志留世正处于大陆地壳俯冲、折返阶段,苦水泉英云闪长岩为俯冲洋壳变质的榴辉岩在陆壳折返阶段发生部分熔融的产物,细粒闪长岩起源于古老的玄武质下地壳的部分熔融。分布在细粒闪长岩上下盘的矿体品位通常远高于平均品位,说明细粒闪长岩为金矿化提供了热动力和热液,也可能提供了部分成矿物质,使得矿体的品位局部变富,由此近似的将细粒闪长岩的年龄作为苦水泉金矿的成矿时代(~428Ma)。苦水泉金矿成矿时代和构造背景的确定,指示柴北缘在早志留世陆壳折返阶段存在一期金矿化。

The Kushuiquan gold deposit,located in the middle part of the North Qaidam structural belt,is a newly discovered gold deposit in recent years,which has the characteristics of the orogenic gold deposit,and whose ore bodies are distributed in tonalite along the fault structure and are closely related to fine-grained diorite vein in space.In this article,investigations in terms of geochemistry,zircon U-Pb dating and Hf isotope data have been carried out on the tonalite and the fine-grained diorite in the Kushuiquan gold deposit.Bulk-rock analyses show that the tonalite is similar to adakite,for it is rich in Na and poor in K(Na2 O/K2 O=6.24~13.09),high in Sr and low in Y(Sr/Y=205~335),which characteristics are also very similar to the adakitic felsic veins within the Xitieshan eclogite.The fine-grained diorites are rich in Al,Ca and Fe,but depleted in Mg.Also,they are enriched in light rare earth elements(LREEs)and large ion lithophile elements(LILEs),and depleted in high field strength elements(HFSEs)and low in Ni and Co.It is thus a typical rock formed in the lower continental crust.Zircon U-Pb dating shows that tonalite and fine-grained diorite were formed at 429.9±2.5 Ma and 428.0±1.9 Ma,respectively.Hf isotopic analysis shows that the zirconεHf(t)values of the tonalite are from+9.3 to+11.9,and the two-stage model age(tDM2)is from 613 Ma to 747 Ma,the zirconεHf(t)values of finegrained diorite are from-31.4 to-9.9,and the two-stage model age(tDM2)is from 1722 Ma to 2803 Ma.The comprehensive analysis shows that the North Qaidam is in the stage of continental crust subduction and exhumation in Early Silurian,and the Kushuiquan tonalite were the products of partial melting of eclogite formed by subduction oceanic crust metamorphism during exhumation of continental crust,and the fine-grained diorite was originated from the partial melting of the ancient basaltic lower crust materials.The grade of the gold orebody distributed in the hanging wall and footwall of the fine-grained diorite is much higher than the average grade,which indicates that the fine-grained diorite provides the thermal power and hydrothermal solution for gold mineralization,and may also provides some ore-forming materials,which makes the grade of gold ore body locally rich.Therefore,the age of fine-grained diorite is approximately regarded as the metallogenic age of Kushuiquan gold deposit(~428 Ma).The determination of the metallogenic age and tectonic setting of the Kushuiquan gold deposit indicates that there is a period of gold mineralization during the exhumation of continental crust in North Qaidam in Early Silurian.